1. Field of the Invention
The present invention relates to a water cooled finger for a pre-heater of a furnace top. When metal scraps are melted in, for example, a melting furnace such as a burner furnace and an arc furnace, it is sometimes common to previously pre-heat metal scraps with exhaust gas of high temperature generated in the melting furnace.
2. Description of the Related Art
For pre-heating metal scraps, various kinds of apparatuses are used. Attention has recently been paid to such a pre-heating apparatus installed at the melting furnace top, which is provided with a shaft at the top of the melting furnace and holds metal scraps charged in the shaft by means of the water cooled finger so as to pre-heat them with the exhaust gas of high temperature issued from the melting furnace.
Conventionally, the water cooled finger of the pre-heater of the furnace top as mentioned above is generally formed as a whole to be hollow by welding steel plates (Unexamined Japanese Patent Publication (kokai) No.8-54191, No.8-136163 and No.9-257377). These fingers are constructed by welding upper plates and lower plates to both side plates. These fingers are formed as a whole to be hollow structures, and the outer configuration in cross section being square. Generally, the hollow part is partitioned, so that cooling water is caused to flow successively into each of the partitioned chambers. However, such a conventional finger is involved with problems about concurrent satisfactions of strength and durability against repetition of dropping impact of charged metal materials and characteristic durable against repetition thermal stress.
Since metal materials are thrown into the shaft through the upper inlet thereof, the finger holding thrown metal materials is repeatedly affected by considerable dropping impact of metal materials. Accordingly, the finger is required to have enough strength and durability against the repetition of dropping impacts. On the other hand, the finger holding thrown metal materials is heated outside by high temperature exhaust gas coming from the furnace, and is cooled inside by the cooling water running in the hollow part of the finger, so that thermal stress caused by heating and cooling acts thereon. Thus, the water cooled finger is also at the same time required to have durable characteristics against repeated thermal stress. However, if the water cooled finger were made of steel plates of relatively large thickness, since the lower plate of the finger exposed to high temperature exhaust gas is affected by considerable thermal stress, cracks would easily appear in the lower plate. Thermal stress is, in its nature, concentrated in welded parts between both side plates at both ends of the lower plate of the finger and the-lower plates thereof, so that cracks easily occur in particular at the welded parts. When cracks appear, the cooling water leaks there, repairs of the finger are frequently required, and affected portions should be exchanged in short periods of time. On the other hand, if he water cooled finger were made of steel plates of relatively mall thickness for satisfying characteristic against repeated thermal stress, since the upper plate thereof directly receiving thrown metal materials is affected by large dropping impact, the upper plate would be easily broken. Similarly in this case, repairs of the finger are also frequently required, and affected portions should be exchanged in short periods of time, after all. In the conventional water cooled finger, it is not possible to concurrently satisfy the need for durable strength against the repeated dropping impacts and the characteristic for durability against the repeated thermal stress.